With an automatic transmission mounted in a vehicle or the like, by the driver of the vehicle operating a select lever in the cabin to select a target range, an operation for establishing a gear stage that corresponds to the selected target range is performed.
According to conventional technology, the select lever and a manual shaft are mechanically coupled via a cable, and when the select lever is operated, the manual shaft is driven for rotation via the cable in association with the operation, and a manual valve is shifted to a position selecting an appropriate range position (P, R, N or D) in association with the rotational movement of the manual shaft. Hydraulic pressure control for causing an appropriate frictional engagement element in a gearshift mechanism unit in the automatic transmission to be engaged or disengaged is performed according to the range position of the manual valve, whereby an appropriate gear stage in the automatic transmission is secured.
In recent years, a shift-by-wire gearshift control system has been conceived (see, for example, Patent Literature 1). With this gearshift control system, by causing the select lever and the manual valve to be uncoupled and a switch operation to be performed with the select lever, a target range signal is output, and the manual shaft is driven for rotation by an actuator in response to the target range signal. Shifting the manual valve to a range position (P, R, N or D) corresponding to the target range in association with the rotational movement of the manual shaft, and performing hydraulic pressure control for causing an appropriate frictional engagement element in the gearshift mechanism unit of the automatic transmission to be engaged or disengaged according to the range position of the manual valve are the same as described above.
The frictional engagement elements are generally clutches and brakes, and configured to be engaged by supply of engaging hydraulic pressure and disengaged by draining the engaging hydraulic pressure. In the hydraulic control circuit, various types of valves are used to supply or drain engaging hydraulic pressure to and from the frictional engagement elements. In the case of draining engaging hydraulic pressure while the engaging hydraulic pressure is being supplied to a frictional engagement element, the engaging hydraulic pressure supplied to the frictional engagement element is caused to flow in the reverse direction to the supply path.
With the shift-by-wire gearshift control system mentioned above, there is a possibility that, for example, an unintentional operational failure or the like in the actuator or the manual valve might cause a switching anomaly in which the target range (target gear stage) corresponding to a driver operation of the select lever and the actually established gear stage (actual gear stage) established in the automatic transmission do not match. If such a switching anomaly occurs when, for example, switching from the forward gear stage to the reverse gear stage, or vice versa, the vehicle may move in a direction opposite to the target direction.
To address the occurrence of switching anomalies, the conventional technique described in Patent Literature 1 mentioned above cuts off a power transmission path from the engine to the driving wheels. In order to cut off the power transmission path, supply of engaging hydraulic pressure is stopped and the engaging hydraulic pressure is drained so as to disengage the frictional engagement elements (clutches and brakes) involved in establishing the actual gear stage.